Bonding Properties of the 1,2-Semiquinone Radical-Anionic Ligand in the [M(CO)4-n(L)n(DBSQ)] Complexes (M = Re, Mn; DBSQ = 3,5-di-tert-butyl-1,2-benzosemiquinone;n= 0, 1, 2). A Comprehensive Spectroscopic (UV−Vis and IR Absorption, Resonance Raman, EPR) and Electrochemical Study

Publisher Website
Google Scholar
Abstract
Rhenium and manganese complexes of the 3,5-di-tert-butyl-1,2-benzosemiquinone (DBSQ) ligand, [M(CO)4(DBSQ)], fac-[M(CO)3(L)(DBSQ)], and cis,trans-[M(CO)2(L)2(DBSQ)], with a widely varied nature of co-ligand(s) (L = THF, Me2CO, MeC(O)Ph, py, NEt3, Ph3PO, SbPh3, AsPh3, PCy3, P(OPh)3, PPh3, dppe-p, PPh2Et, P(OEt)3, PEt3) were generated in solution and characterized as valence-localized molecules containing the radical-anionic DBSQ ligand bound to ReI or MnI metal atoms. This is evidenced by the following. (i) Carbonyl stretching frequencies ν(C⋮O) and average force constants kav are typical for MnI or ReI carbonyls. (ii) Frequencies of the intra-dioxolene CO bond stretching vibration, ν(CO), lie within the 1400−1450 cm-1 range which is diagnostic for coordinated semiquinones. (iii) EPR spectra indicate a very small spin density on the metal atom (0.2% < aM/Aiso > 2.6%). (iv) Absorption spectra show ReI → DBSQ MLCT electronic transitions characterized by a resonant enhancement of the Raman peaks due to the ν(C⋮O) and intra-DBSQ ν(CO) vibrations. (iv) Finally, the electrochemical pattern consists of DBSQ/DBQ and DBSQ/DBCat ligand-localized redox couples. All these properties are, in a limited range, dependent on the nature and, especially, the number of co-ligands L, indicating a small delocalization of the singly occupied MO of the DBSQ ligand over the metal atom. The extent of this delocalization may be finely tuned by changing the co-ligands, although in absolute terms, it remains rather limited, and the DBSQ ligand behaves toward ReI and MnI as a very weak π-acceptor only. The changes of the electronic properties of the metal center induced by the co-ligands are mostly compensated by more flexible M → CO π back-bonding as is manifested by large variations of the average C⋮O stretching force constant.